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Myself writes: “So, you’ve sealed all the gaps in your home’s skin, added insulation in all the right places, and maybe taken the plunge on some new windows.  An energy audit is the next step to efficiency, and the Kill-A-Watt is an affordable way to get started.

“It shows you the voltage and frequency that the power company is delivering, and measures the amount that your appliance draws.  It tells you amps, watts, Volt-amps, and power factor.  (At DC, watts and VA are the same thing, but in AC, a phase lag between the voltage curve and the current curve causes non-unity power factor, which is why your computer’s UPS might be rated to drive 700VA but only 450 watts.)  Having this information is essential for UPS sizing, energy planning, and circuit troubleshooting.”

“For data geeks, there’s also the Watts-Up, a more expensive instrument that includes more advanced datalogging: http://www.doubleed.com/

Froogle turns up the Kill-a-Watt for \$20 to \$25 all over, though Myself noted that it’s available via a place called “SuperMediaStore” for \$19 (\$24 minus \$5 with the code “silverbells”) and free shipping.  That’d beat the deals we found.

The Kill-a-Watt [P3 International]
Street Pricing [Froogle]

### 3 Responses to Reader Find: The Kill-a-Watt

1. SuperJdynamite says:

“but in AC, a phase lag between the voltage curve and the current curve causes non-unity power factor, which is why your computer’s UPS might be rated to drive 700VA but only 450 watts.”

That’s actually dependant on load. In some applications, such as a light bulb, current and voltage would be in phase.

2. Myself says:

Whoops, I sure wasn’t very clear was I? Of course the degree of phase lag depends on the load’s reactive component, and a purely resistive load will have none at all. Thanks for catching that.

Most home users will never need to know their power factor anyway. Just generously oversize any UPS purchases and it’s not an issue. Commercial power customers, on the other hand, are usually billed for reactive power, so they definitely want their switchmode power supplies to be power-factor corrected, etc.

3. [...] Here’s another Toolmonger item from way back that’s a good deal right now: Meritline has the Kill-A-Watt listed for \$20 right now — about \$2-\$3 cheaper than normal – and while we did find it for \$18 from one other mail-order-only shop, Meritline beats ‘em out on shipping.  [...]